Coordinate measuring machines are used for dimensional inspection of workpieces such as machined parts. A workpiece is typically secured to a table, and a measuring probe is movable within a measuring volume. Measuring scales monitor the position of the probe within the measuring volume. To measure the coordinates of a point on the workpiece, the probe is brought into contact with the point, and the X, Y and Z measuring scales of the machine are read. To measure a distance between two points, the points are contacted successively, the coordinates of both points are read and distance is calculated from the coordinates. State of the art coordinate measuring machines have refinements such as high resolution measuring systems, electrical contact probes, motor drives, computer control drives and computer acquisition and processing of data.
Conventional moving bridge coordinate measuring machines (CMM) include a bridge that moves in the Y direction along guideways on the table of the CMM. A carriage moves in the X direction along guideways on the bridge. A ram with a probe mounted to its lower end moves vertically through bearings in the carriage. Scale systems between the bridge and the table, between the carriage and the bridge, and between the ram and the carriage indicate the positions of the movable elements in three orthogonal directions. A moving bridge CMM is disclosed, for example, in U.S. Pat. No. 4,939,678, issued Jul. 3, 1990 to Beckwith, Jr. Another prior art coordinate measuring machine is the horizontal arm machine, wherein a horizontal ram is supported by a Z carriage on a Z rail.
Prior art coordinate measuring machines provide highly satisfactory performance, in terms of both accuracy and reliability. However, such systems are relatively costly because of their complexity and because of the need for precision mechanical parts, such as the guideways upon which precision air bearings carry a carriage or a ram. It is desirable to provide coordinate measuring machines which are highly accurate and reliable, yet are simple in construction and low in cost.
Various pivoting manipulators and articulated arms have been disclosed in the prior art. Japanese Document No. 3-100416, published Apr. 25, 1991, discloses apparatus for determining the position of a detecting probe which utilizes a pivoting linkage. U.S. Pat. No. 4,341,502, issued Jul. 27, 1982 to Makino, discloses an assembly robot which includes a quadrilateral link mechanism for controlling the position and the attitude of a workpiece. U.S. Pat. No. 5,180,955, issued Jan. 19, 1993 to Karidis et al, discloses positioning apparatus which includes a multibar linkage drive mechanism. U.S. Pat. No. 4,329,111, issued May 11, 1982 to Schmid, discloses a mechanical manipulator including an articulated arm having a first member pivotally connected to a first pivot axis and a second member pivotally connected at a second pivot axis to the first member. U.S. Pat. No. 4,894,595, issued Jan. 16, 1990 to Sogawa et al, discloses an industrial robot having a movable arm with a support element which maintains a parallelogram shape throughout the range of motion of the arm. U.S. Pat. No. 3,703,968, issued Nov. 28, 1972 to Uhrich et al, discloses a manipulator arm having two parallelogram linkages in combination with a trapezium linkage. Insofar as known to applicants, prior art coordinate measuring machines have not utilized pivoting mechanical structures for positioning a measuring probe. A significant problem in applying pivoting mechanical strucutures for positioning a probe in a coordinate measuring machine is that of measuring the angles of the pivoting structure with the required precision at an acceptable cost.